1. Field of the Invention
The present invention relates to a method of calibrating an inertial unit having accelerometers and angle sensors.
2. Brief Discussion of the Related Art
In general, an inertial unit has three accelerometers and three angle sensors of the gyro type mounted on a frame. The accelerometers and the angle sensors have axes that are substantially in alignment with the axes of a measurement frame of reference and they are connected to a control unit that serves to acquire measurement signals from the accelerometers and the angle sensors and to process them, at least in part.
Accelerometers and sensors present defects, such as biases and drifts, and it is necessary to know them in order to take them into account when processing the measurement signals so as to improve the accuracy of the inertial unit.
For this purpose, it is common practice to calibrate the inertial unit before using it.
Calibration seeks to make it possible to estimate some or all of the biases and drifts and to determine a set of parameters for processing the measurement signals so as to make it possible to minimize these biases and drifts, in full or in part. Calibration is particularly useful for correcting errors associated with accelerometer biases since they are practically constant over short periods of use.
Various calibration methods exist.
The most widespread method consists in aligning the sensing axes of the accelerometers and of the angle sensors in succession on the vertical in order to cause each accelerometer to measure the acceleration due to apparent gravity, and to deduce a sensor error therefrom.
That extremely simple calibration technique is nevertheless not possible when the inertial unit is mounted on a support that does not allow for the various orientations needed for performing the calibration.
Another method consists in using a reference inertial unit that is maintained in the same position as the inertial unit for calibration and in determining the set of processing parameters that enables the inertial unit for calibration to deliver measurements that are as close as possible to those delivered by the reference inertial unit.
The set of processing parameters is selected to minimize at least some of the errors of the inertial unit for calibration, however it is affected by errors of the reference inertial unit and by errors in positioning the reference inertial unit relative to the inertial unit for calibration.